In a conventional magnetic substance detection device, a technique is generally known for detecting a magnetic substance, such as a magnetic ink contained in a bank bill, by combining a magnetoresistive element with a permanent magnet for generating a bias magnetic field. Since a very small amount of the magnetic substance is contained in the bank bill, reading is generally performed in a state where the bank bill is brought into contact with a magnetic sensor head which is composed in combination with the magnetoresistive element and the permanent magnet.
In Patent Document 1 (FIGS. 7 and 8), on the other hand, a pair of magnetic sensor heads are arranged in opposition to each other to allow a material to be detected, such as bank bill, to pass through a gap between both the heads, so that non-contact reading is performed. Moreover, in Patent Document 1, as a magnetoresistive element that detects strength of a magnetic field, a semiconductor magnetoresistive (SMR) element is used. Since a direction in which the semiconductor magnetoresistive element can sense the magnetic field (i.e., magnetosensitive direction) is perpendicular to a detection surface of the element, as to a magnetization direction of a permanent magnet in Patent Document 1, different poles are opposed to each other to apply a bias magnetic field in the direction perpendicular to the detection surface of the element.
Moreover, in Patent document 1, two semiconductor magnetoresistive elements MR1 and MR2 make up a half bridge circuit to extract a midpoint potential for an output signal, so that a change in resistance value of the semiconductor magnetoresistive element that may be caused due to a change in temperature, i.e., output offset fluctuation can be cancelled.